1. Field of the Invention
The present invention relates to an electrostatic latent image developing toner that is used when an electrostatic latent image formed by an electrophotography method or an electrostatic recording method is developed with a developer and a manufacturing method thereof.
2. Description of the Related Art
A method of visualizing image information through an electrostatic latent image such as an electrophotography method is presently used in various fields. In the electrophotography method, an electrostatic latent image is formed on a photoreceptor by charging and exposure and visualized through developing the electrostatic latent image with a developer including an electrostatic latent image developing toner (hereinafter, in some cases, simply referred to as “toner”), transferring and fixing.
As a developer that is used here, a two-component developer made of a toner and a carrier and a single-component developer that singly uses a magnetic toner or a non-magnetic toner are known. As a manufacturing method of the toner, normally, a kneading-pulverizing process in which a thermo-plastic resin is melted and kneaded together with a pigment, a charge control agent and a releasing agent such as wax, followed by pulverizing and classifying after cooling is used.
In some cases, as need arises, in order to improve the fluidity and the cleaning property, inorganic or organic particles are added to a surface of a toner particle. The method can produce a rather excellent toner; however, since a toner shape is substantially limited to an amorphous one, particles tend to be generated, and a releasing agent and a colorant are likely to be exposed on a surface, whereby in some cases, problems such as deterioration of the developability and image quality due to stress in a developing device, contamination to other members and so on may be caused.
In recent years, as a process that makes it possible to intentionally control a shape and a surface structure of a toner, a manufacturing process of toner according to an emulsion-polymerization flocculation method has been proposed (for example, see JP-A Nos. 63-282752 and 06-250439). This is a manufacturing method in which, in general, after a dispersion of resin particles is prepared by means of emulsion polymerization and a colorant dispersion in which a colorant is dispersed in a solvent is prepared, these are blended to form flocculated particles corresponding to a toner particle diameter, followed by heating to melt and unite the same. According to this method, to some extent, the toner shape can be controlled and the charging property and the durability can be improved; however, an internal structure becomes substantially homogeneous. Accordingly, there still remain problems in the peeling property of toner receiving bodies at the fixing, the stability of the transparency at the time of OHP output, and the difference in charge amount between colors in the color toner.
Thus, in the electrophotography process, in order for the toner to stably maintain the performance even under various mechanical stresses, it is necessary to suppress exposure a releasing agent at the surface, enhance the surface hardness and improve the mechanical strength of the toner itself without damaging the fixing property, and combine sufficient charging property and fixing property.
Furthermore, from the viewpoints of recent high-speed processing and ensuring low energy consumption, a toner having uniform charging property, maintainability, toner strength and a narrow particle size distribution becomes more and more important. Furthermore, in view of the high-speed processing and power saving properties of these machines, the low temperature fixing property becomes more and more necessary.
From the viewpoint of the foregoing low temperature fixing, as a method of lowering a fixing temperature of the toner itself, a technology in which a glass transition temperature (Tg) of a binder resin for toner is lowered is generally adopted. However, since powder tends to aggregate (blocking) and the storage stability of the toner on a fixed image is lost when the Tg is excessively lowered, from a practical standpoint, the lower limit is 60 degrees centigrade. This glass transition point is a design point of many of commercially available toner resins. That is, there is a problem in that, according to a method of lowering the glass transition point, a toner that can be fixed at a temperature lower than that which is currently possible cannot be obtained. When a plasticizer is used, the fixing temperature can be lowered; however, there is a problem in that during storage of the toner or in a developing device, blocking occurs.
As a measure that combines inhibition of blocking, image storage stability up to 60 degrees centigrade and low temperature fixing property, a technology that uses a crystalline resin as a binder resin that constitutes the toner has been known from long ago (for example, see JP-B No. 56-13943). Furthermore, for the purpose of offset prevention, pressure fixing and so on, a technology that uses a crystalline resin has been known from long ago (for example, see JP-B Nos. 62-39428 and 63-25335). However, according to the foregoing disclosed technologies, since a melting point of a resin that is used is excessively low such as 62 to 66 degrees centigrade, there are problems in the reliability of powder and images and insufficient fixing property of the crystalline resin onto paper.
As a crystalline resin that is expected to improve the fixing property onto paper, a polyester resin can be cited. As a technology that uses a crystalline polyester resin in toner, there is a technology in which a non-crystalline polyester resin having a glass transition point of 40 degrees centigrade or more and a crystalline polyester resin having a melting point in the range of 130 to 200 degrees centigrade are blended and used (for example, see JP-B No. 62-39428). However, this technology, though having excellent pulverizing property and blocking resistance, cannot achieve a low temperature fixing property beyond that achieved conventionally since the melting point of the crystalline polyester resin is high.
In order to overcome the foregoing problems, a technology that uses a toner in which a crystalline resin having a melting point of 110 degrees centigrade or less and a non-crystalline resin are blended is proposed (for example, see JP-B No. 04-30014). However, when a non-crystalline resin is blended with a crystalline resin, there are practical problems in that lowering of the melting point of the toner is caused, toner blocking is caused, and the storage stability of images is deteriorated. Furthermore, when the non-crystalline resin component is contained in a large amount, since the characteristics of the non-crystalline resin are largely exhibited, it is difficult to lower the fixing temperature more than in conventional technologies. Accordingly, there is a problem in that unless the crystalline resin is used singly, if mixed, with only a slight amount of the non-crystalline resin, as the toner resin, practical use is difficult.
Furthermore, as the technologies that use a crystalline polyester resin, some proposals have been made (for example, see JP-A Nos. 04-120554, 04-239021 and 05-165252). In these technologies, the crystalline polyester resin is a resin that uses a carboxylate component of terephthalic acid and alkylene glycol or alicyclic alcohol having a small number of carbon atoms. These polyester resins, though described as crystalline polyester resins in the foregoing literature, are substantially partially crystalline polyester resins. As a result, a change in viscosity of the toner (binder resin) relative to a temperature is not steep, and accordingly, there is no problem in the blocking property and the storage stability of image, however, in heat roll fixing, low temperature fixing cannot be realized.
Still furthermore, a toner that contains as a main component a crystalline polyester resin having a cross-linking structure has been shown to be excellent in blocking resistance and image storage stability, and capable of realizing low temperature fixing (for example, see JP-A No. 2001-117268). However, there is a problem in that in oil-less fixing the peeling property is not stable. In addition, when a crystalline resin is used singly, though the low temperature fixing, the toner storage property, and the document storage stability can be assuredly improved, there are problems in that the strength of a developed image is low and an image can be easily damaged by scratching and so on.
Furthermore, there is a problem in that when double-side printing is carried out with the foregoing toners by use of a copy machine or a printer, in particular when one side is printed with a solid image and the other side is printed with a half-tone image, since the fixing property differs largely, paper printed with images warps (so-called curling).
Still furthermore, from the viewpoint of improving the fixing property in a high-speed and low-pressure fixing system, it is disclosed that an improvement in the oil-less stripping property owing to an improvement in the dispersing property of the releasing agent inside of the toner is achieved by defining the relaxation elasticity and the relaxation time obtained from dynamic viscoelasticity measurement (for example, see JP-A No. 2000-81721). According to this method, since a behavior of the toner at the fixing relates to the deformation of toner particles in a fixing system and a stress relaxation phenomenon thereof and, among these, to a change of state of the toner from a glass state to a molten state affected by a temperature, certainly the fixing characteristics and the stress of a fixed image derived from an internal structure of the toner, which cannot be sufficiently controlled by the storage elasticity and loss elasticity that have so far generally been used as parameters or the loss tangent which is a ratio thereof, can be accurately controlled to a certain degree, whereby the internal stress can be reduced. However, according to this method, in some cases, problems such as dependence of the fixing property on the process (for instance, a process speed) and warping of an image, particularly in the case of thin paper, when double-sided printing is applied cannot be avoided. Accordingly, practical application of a toner that can further eliminate these problems is demanded.